Bring r21771 over from Five's 1.3 branch: Five 1.3 is released.

r21753 | efge | 2006-01-06 18:58:06 +0100 (Fri, 06 Jan 2006)
Fix cleanup of five:traversable.

r21752 | regebro | 2006-01-06 18:51:19 +0100 (Fri, 06 Jan 2006)
If one class was set to have a localsite hook twice, removing the hook
would be attempted twice during the cleanup of unit tests, and the tests
would fail.